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Paleoseismology of blind-thrust faults beneath Los Angeles, California: implications for the potential of system-wide earthquakes to occur in an active fold-and-thrust belt

PALEOSEISMOLOGY OF BLIND-THRUST FAULTS BENEATH LOS ANGELES,
CALIFORNIA: IMPLICATIONS FOR THE POTENTIAL OF SYSTEM-WIDE
EARTHQUAKES TO OCCUR IN AN ACTIVE FOLD-AND-THRUST BELT
by
Lorraine Annette Leon
A Dissertation presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(GEOLOGICAL SCIENCES)
August 2009
Copyright 2009 Lorraine Annette Leon

In order to understand the paleo-earthquake history and structural evolution of blind-thrust faults and their associated folds, I use a multi-disciplinary methodology to link blind faulting at seismogenic depths directly to near-surface fault-related folding. My research focused on two major blind-thrust systems beneath metropolitan Los Angeles, California; the Puente Hills thrust fault (PHT) and the Compton thrust fault. I acquired data from multiple study sites along both faults in order to analyze shallow, subsurface folding for individual thrust ramps and study the potential for system-wide earthquakes to occur in segmented blind-thrust systems.; To resolve how folds grow in response to slip on the underlying thrust ramps, I utilized a methodology that combines high-resolution seismic reflection profiles, borehole excavations, and cone penetration testing to analyze the overlying growth folds of segmented blind-thrust faults. These shallow data allow me to observe folding at multiple depths and identify discrete buried fold scarps formed during past earthquakes at study sites above individual thrust ramps of both the PHT and Compton thrust.; The results of this dissertation provide Holocene-Late Pleistocene slip rates from study sites above the Los Angeles, Santa Fe Springs, and Coyote Hills, segments of the PHT. I also determined slip rates for both the northern and southern segments of the Compton fault. In addition to fault slip rates, this research demonstrates that both the Puente Hills and Compton blind thrust faults have generated multiple, large-magnitude (Mw >7) earthquakes during the past 12-14 ka, and that both faults are capable of generating multi-segment ruptures. Moreover, this study provides insights into the detailed kinematics of earthquake-by-earthquake fold growth above the underlying blind thrust ramps. At all of the study sites, the borehole data show that the folded strata within the kink bands acquired their dips incrementally, suggesting that fold kinematics involves components of both kink-band migration and limb rotation. These analyses of the geometry and evolution of young, shallow growth structures have allowed me to decipher the previously unknown paleo-earthquake history of the PHT and Compton fault, providing the basis for effective seismic hazard analysis.

PALEOSEISMOLOGY OF BLIND-THRUST FAULTS BENEATH LOS ANGELES,
CALIFORNIA: IMPLICATIONS FOR THE POTENTIAL OF SYSTEM-WIDE
EARTHQUAKES TO OCCUR IN AN ACTIVE FOLD-AND-THRUST BELT
by
Lorraine Annette Leon
A Dissertation presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(GEOLOGICAL SCIENCES)
August 2009
Copyright 2009 Lorraine Annette Leon